Electromagnetic interference (EMI) shielding materials are well known in the art in forms such as gaskets, caulking compounds, adhesives, coatings and the like for a variety of EMI shielding purposes.
In the past, Where high shielding performance is necessary, EMI shielding has tended to use silver particles or silver coated copper particles dispersed in a resin binder. More recently, aluminum core silver coated particles as said forth in U.S. Pat. No. 4,507,359 have been used to reduce costs while maintaining good electrical and physical properties.
Prior art particles have often exhibited disadvantages which include one or more of, limited high-temperature functioning without loss of electrical conductivity, corrosion of particles internally or externally of gaskets, or in the case of silver coated glass particles, limited current carrying capacity in cases where electromagnetic pulse conditions are encountered.
Other U.S. patents showing the state of the art include U.S. Pat. Nos. 3,140,342; 3,194,860; 3,202,488; 3,476,530; 3,583,930 and 4,434,541; as well as published Hungarian Pat. Document No. 166,727 (referred to in Chemical Abstract No. 159606S).
The present inventions provides an EMI shielding material which avoids high cost of pure silver particles while providing good EMI shielding effectiveness. The present particles when incorporated in shielding, form end structures which provide desirable features in shielding and are even more effective in corrosion resistance than previous particles including the substantially improved relatively recently developed aluminum core precious metal coated particles of the prior art. Maintaining low resistance or impedance across an EMI joint over the life of an electronic enclosure is critical for shielding. Corrosion of the conductive particles (internal corrosion) or of the flange material at the joint with a shield such as a gasket (external corrosion) can be a substantial problem. The particles of the invention in addition to increased corrosion resistance, maintain advantages of such prior art aluminum core particles which include reduction in weight and cost which are particularly significant in certain applications including areospace applications.